In most manufacturing processes, it is generally desirable to control as many variables as practical, so as to maintain product consistency, maximize throughput, and provide other advantages. For example, in a heat-curing process in which a product is conveyed through an oven, oven temperature is generally controlled by monitoring oven temperature and feeding back a temperature signal to a controller, which adjusts the oven heat source.
As illustrated in FIG. 1, in conventional manufacturing of soft (hydrophilic polymer) contact lenses, a liquid polymer material is injected into molds 10, which are then transported on a conveyer belt 12 through a curing chamber 14. Tube-shaped fluorescent lamps 16 that emit ultraviolet (UV) radiation are mounted in curing chamber 14. Subjecting molds 10 to UV radiation as they pass through curing chamber 14 cross-links and thus toughens the polymer. Although lamps 16 are shown mounted only above conveyer belt 12 for purposes of illustration, it should be recognized that a conventional curing chamber can include lamps 16 mounted in various positions, including some lamps 16 above conveyer belt 12 and others below conveyer belt 12, to more evenly expose molds 10 to the UV light. Curing chamber 14 is sometimes referred to in the art as an oven, though the primary source of the curing effect is UV light rather than heat.
Lamps 16 are powered through power supply circuitry 18 that includes ballast 20. As a fluorescent lamp 16 is a negative-resistance device, i.e., its resistance drops as more current flows (thus allowing still more current to flow), ballast 20 is needed to moderate the current through the tube. Although some types of ballasts for fluorescent lamps can include active electronics, such as power transistors and digital circuitry, ballast 20 can be as straightforward as a single inductor or capacitor. Ballasts 20 are conventionally mounted on or in the same fixtures 22 in which lamps 16 are mounted. As ballasts 20 produce waste heat, a blower or fan 24 is included in chamber 14 to draw cooling air through chamber 14 and prevent over-heating. Although ballasts 20 are shown in FIG. 1 for purposes of illustration as being exposed (to the interior of chamber 14), in other conventional curing chamber arrangements the ballasts may be enclosed inside the fixtures and thus somewhat more thermally isolated from the interior of chamber 14. In such an arrangement, the ballast-generated heat may be conducted through the fixture walls into the chamber.
When power supply circuitry 18 energizes an electrode (not separately shown) in each lamp 16, the electrical energy emitted at the electrodes excites the gas with which the lamp tube is filled, causing it to transform to a plasma state. The plasma produces short-wave UV light. The interior of each lamp tube is coated with a phosphor. The UV excites the phosphor, causing it to fluoresce and thus produce visible light. The relative proportions of UV light and visible light emitted from the tube are functions of the phosphor material and the tube's transmissibility (i.e., tubes can incorporate filters or be made from doped glass). Lamps 16 that are to be used primarily as sources of UV light have a phosphor that promotes emission of a substantial proportion of UV light and a lesser proportion of visible light. As much of this visible light is close to UV, lamps 16 generally can be observed as emitting a purple or violet light. Such UV lamps 16 are used not only in industrial curing chambers but also to illuminate artwork or other ornamental objects having surfaces that fluoresce when exposed to UV light. Indeed, the same UV lamps 16 are typically used in both industrial curing and ornamental illumination applications, as they have generally been deemed adequate for both applications.
The light intensity or power emitted by a fluorescent lamp is generally believed in the prior art to be stable and constant, once the lamp has warmed up to a stable operating temperature. (Power, also referred to as intensity, is typically quantified in such an instance in units of milliwatts per square centimeter.) For example, manufacturers of lamps 16 generally specify that each lamp 16 will emit UV radiation (also referred to herein as UV light) at a specified intensity (mW/cm2) when operated at a specified voltage and temperature. It is generally believed in the art that lamps 16 should optimally be allowed to operate for about 20-30 minutes, thereby allowing chamber 14 to warm up somewhat, before beginning to convey molds 10 through chamber 14. As fluorescent lamps do not themselves generate much heat, the warming of chamber 14 is primarily due to heat generated by ballasts 20.
Once chamber 14 has been warmed up, and molds 10 begin passing through chamber 14, it generally has been presumed that the UV light that impinges upon molds 10 as they pass through chamber 14 is sufficient to effect curing, and little further control is exercised over the curing chamber process other than to maintain conveyer belt 12 at a substantially constant speed and lamps 16 at a substantially constant voltage. As ballasts 20 can become quite warm, fan 24 is typically continuously operated to continuously draw air through chamber 14 and thereby minimize the likelihood of overheating. Such airflow is indicated by heavy arrows in FIG. 1, with air entering chamber 14 at intake ports adjacent conveyer belt 12 and exiting or exhausting through fan 24. As noted above, the arrangement shown in FIG. 1 is intended only to be exemplary of a conventional UV curing chamber of the type used in contact lens manufacturing, and other arrangements are also known.